Up to now, the main display apparatus has been CRT. Instead of the CRT, a liquid crystal display apparatus or a plasma display apparatus which are flat display apparatuses are put into practical use, and increasingly demanded. Also, in addition to those display apparatuses, a display apparatus (hereinafter referred to as “organic EL display apparatus (OLED)”) using organic electro luminescence and a display apparatus (FED display apparatus) in which electron sources using field emission are arranged in a matrix, and illuminate phosphors that are arranged on anodes to form an image are increasingly developed and put in practical use.
The organic EL display apparatus has the following features. (1) No backlight is required because the organic EL display apparatus is of the light emitting type as compared with liquid crystal. (2) There is the possibility that the power consumption can be reduced because a voltage required for light emission is 10 V or lower. (3) The organic EL display apparatus is suitable for lightweight and thinning because no vacuum structure is required as compared with the plasma display apparatus and the FED display apparatus. (4) The organic EL display apparatus is excellent in the moving picture characteristic because a response time is several microseconds which is short. (5) A viewing angle is 170 degrees or more which is wide.
Because the organic EL display apparatus can be lightened in weight and thinned, the device can be widely used also as a portable display apparatus. In order to keep the viewability of a screen even outdoors where outside light is strong, it is necessary to display the screen with high luminance. On the other hand, a normal display mode can be applied in use indoors. From the viewpoint of the power consumption, it is desirable that the display mode can be changed over.
FIG. 11 shows a relationship between the voltage and the luminance of an OLED element 1. The emission luminance of the OLED element 1 increases more as an applied voltage is higher. When a supply voltage increases, a current that flows in the OLED element 1 increases, and the recombination of electrons with holes is increased to enhance the emission intensity. Accordingly, a display apparatus having plural maximum luminance, that is, a display apparatus that can change over the display mode is generally realized by changing the supply voltage.
On the other hand, the organic EL display device using a thin film transistor (TFT) is excellent in image quality such as contrast. However, when gradation display is conducted, the display characteristic is varied with being affected by the characteristic variation of the respective TFTs. As an example of the conventional art that copes with the above drawback, there is a technique shown in FIGS. 12 to 14, which is called “first conventional example” in the present specification.
FIG. 12 shows a driver circuit for a pixel portion in the first conventional example. In FIG. 12, an OLED element 1, a lighting TFT switch 2, and an OLED driving TFT 3 are connected in series between a power supply line 51 and a reference potential. In this example, the reference potential is a potential that is a reference of the display apparatus, which is a broad concept including an earth potential. In FIG. 12, the lighting TFT switch 2 is a switch for determining whether a current is made to flow in the OLED element 1, or not. The OLED driving TFT 3 controls the current that flows in the OLED element 1, and determines the gradation of the emission of the OLED element 1. The image data is written in a retention volume 4 from a signal line 54.
When a threshold voltage Vth of the OLED drive TFT 3 is varied, it is impossible to precisely conduct the gradation display. A reset TFT switch 5 in FIG. 12 is used in the reset operation for preventing the electric charges of the retention volume 4 which reflect a data signal from being affected by the threshold voltage Vth. The gradation display that precisely reflects the image data can be conducted by the reset operation.
FIG. 13 is a circuit diagram showing the entire display apparatus using the pixels shown in FIG. 12. The screen is formed by a large number of pixels, but FIG. 13 indicates only four pixels. Scanning signals and the data signals are inputted to the respective pixels by means of a timing controller 110.
A gate driver circuit 200 is located in a lateral direction of the screen. Reset lines 52 and scanning output lines 151 extend from the gate driver circuit 200. Each of the reset lines 52 is connected to the gate of a reset TFT switch 5, and each of the scanning output lines 151 is inputted to a lighting switch OR gate 150. A lighting control line 105 is inputted to the lighting switch OR gates 150. A signal is outputted to the gates of the lighting TFT switches 2 from the lighting switch OR gate 150 according to any one of the signals from the scanning output lines 151 or the signals from the lighting control lines 105.
A signal driver circuit 100 is located above the screen. The image signal is supplied to the signal driver circuit 100 from the external through a signal input line 1001. The signal lines 54 extend toward the screen from the signal driver circuit 100. Not only the image data signal but also a chopping wave from a chopping wave generator circuit 111 are inputted to the signal lines 54. The chopping wave is to determine the emission start times of the respective OLED elements 1 on the basis of the data signal.
FIG. 14 shows a timing chart for driving the driver circuit of FIG. 12. The driver circuit divides one frame into a write operation period, an emission period, and a blanking period as shown in the upper portion of FIG. 1. The gradation signal is written in each of the pixels in the write operation period. The write operation position in FIG. 14 shows an appearance in which the data is written in the order of the scanning lines. The lower portion of FIG. 14 shows a write timing of one pixel. In FIG. 14, the reset TFT switch 5 first turns on to short-circuit the gate and source of the OLED drive TFT 3. Thereafter, the lighting TFT switch 2 turns on to allow a current to flow in the OLED. In this state, the inverter can be formed by the OLED element 1 and the OLED drive TFT 3. The gate and source of the OLED drive TFT 3 are short-circuited by the reset TFT switch 5. With the above configuration, the gate potential of the OLED drive TFT 3 is set to a point where the source and gate of the OLED drive TFT 3 become identical in the potential with each other on a characteristic curve that determines the relationship of the gate and source of the OLED drive TFT 3. In this case, the gate potential of the OLED drive TFT 3 is uniquely determined according to the threshold voltage Vth of the OLED drive TFT 3. Hereinafter, the gate potential is called “given potential”. Since the signal voltage is written in the gate potential, it is possible to remove an influence of the variation of Vth on the OLED drive TFT 3. Thereafter, the reset TFT switch 5 and then the lighting TFT switch 2 are turned off. As a result, electric charges that correctly reflect the signal voltage are stored in the retention volume 4 to enable correct gradation electric charges.
After the write operation has been conducted on all of the scanning lines, the period is shifted to the emission period. The chopping wave is inputted to the retention volume 4 during the write period. As a result, the OLED element 1 emits a light according to the potential that is retained in the gate of the OLED drive TFT 3 with a time difference to conduct the gradation display.
Another example that copes with the variation of Vth of the OLED drive TFT 3 is shown in FIGS. 15 to 17. This example is called “second conventional example”. FIG. 15 is a driver circuit of one pixel. In FIG. 15, the OLED drive TFT 3, the lighting TFT switch 2, and the OLED element 1 are connected in series from the power supply line 51. The lighting TFT switch 2 controls whether the light emission of the OLED element 1 is enabled, or not. The OLED drive TFT 3 conducts the gradation display by a voltage that is determined by the electric charges that have been stored in a first retention volume 41 and a second retention volume 42. Similarly, in this case, in order to suppress the emission characteristic of the OLED element 1 from being varied by the variation of Vth of the OLED drive TFT 3, the reset TFT switch 5 is used.
FIG. 16 is a circuit diagram showing the entire display apparatus using the pixels shown in FIG. 15. The screen is formed of a large number of pixels. However, FIG. 16 indicates only four pixels. The scanning signals and the data signals are inputted to the respective pixels by the timing controller 110.
The gate driver circuit 200 is located in the lateral direction of the screen. A select switch line 55, a lighting switch line 53, and a reset line 52 extend from the gate driver circuit 200. The select switch line 55 is connected to the gate of the select switch 6, the lighting switch line 53 is connected to the gate of the lighting TFT switch 2, and the reset line 52 is connected to the gate of the reset TFT switch 5.
The signal driver circuit 100 is located above the screen. The image signal is supplied to the signal driver circuit 100 from the external through the signal input line 1001. The signal line 54 extends from the signal driver circuit 100 toward the screen. The input/output of the signal from the signal line 54 to the pixel is controlled according to the signal line select switch control line 104.
The operation of the driver circuit shown in FIG. 15 will be described with reference to FIG. 17. Since the TFT used in FIG. 17 is of the p-type, the TFT turns on when receiving a negative signal. In the second example, when the gradation voltage is written in each of the pixels, the gradation voltage is maintained for one frame period, and the OLED element 1 emits a light. In FIG. 17, the lighting TFT switch 2 is in an on-state. In this state, the select switch 6 turns on. As a result, it is possible to input data from the signal line 54 to the pixels. Subsequently, when the reset TFT switch 5 turns on, the drain voltage of the OLED drive TFT 3 shown in FIG. 16 and the gate voltage of the OLED drive TFT 3 are forcedly short-circuited. Subsequently, when the lighting TFT switch 2 turns off, the gate potential of the OLED drive TFT 3 converges on a value lower than the supply voltage by Vth of the OLED drive TFT 3. Thereafter, when the reset TFT switch 5 turns off, and the signal voltage is written from the signal line 54, the electric charges that reflect the signal voltage are stored in the second retention volume 42 and the first retention volume 41 regardless of the variation of Vth of the OLED drive TFT 3, to thereby enable the gradation display.
The above techniques are disclosed in JP-A 2003-5709, JP-A 2003-122301, and “Digest of Technical Papers, SID98, pp. 11-14”.